This invention relates in general to forming operations and more particularly to a front end for use in forming operations. Most particularly, this invention relates to an oxygen-fired front end for use in glass forming operations.
In a forming operation, batch material is reduced to molten substance (e.g., molten glass) by passing the batch material through a melting furnace, which is commonly referred to as a melter. The molten glass is delivered downstream from the melter through a system of channels and forehearths. This system of channels and forehearths is referred to as a front end. The front end serves as a conduit to deliver the molten glass to one or more production points. The front end also serves to cool and condition the molten glass prior to reaching the production points. These production points are referred to as forming positions. Each forming position includes a bushing for fiber forming operation or a gob cutter for a container forming operation. Bushings or gob cutters are secured to the forehearths via a forehearth steel.
A conventional forehearth is provided with a firing system, which includes a plurality of burners. The burner functions to condition the molten glass G and maintain the molten glass G at a desired operational temperature. An example of a conventional forehearth 10 is shown in FIG. 1. The forehearth 10 includes a top or crown (not shown), a bottom (also not shown), and laterally spaced sidewalls 16. Portions of the forehearth 10 above the level of the molten glass G are constructed from super structure refractory. Portions of the forehearth 10 below the level of the molten glass G are constructed from contact refractory (i.e., glass contact refractory).
A plurality of holes 18 is drilled through the sidewalls 16. The holes 18 are drilled through the super structure of the forehearth 10. The holes 18 are drilled at a right angle relative to the sidewalls 16. The holes 18 are adapted to receive burners 20. The holes 18 are spaced about four to five inches from one another. Consequently, a large number of burners, manifolds, pipes, fittings and valves (not shown) are associated with air-gas mixture burners.
In a conventional firing system, a source of air and a source of gas pass through regulators. The air and gas are mixed and then passed through a system of pipes to a plurality of burners, typically 20 to 100 burners. The burners are typically air-gas mixture burners. That is to say, the burners use the air as an oxidant for the combustion of the gas to provide heat to a zone, commonly referred to as a control zone. The front end has between six and sixty control zones, each complete with a gas control safety and pressure reduction system, combustion air blowers, and valves and regulators capable of controlling the temperature of the molten glass G between the melter and the forming position.
An air-gas mixture firing system is not only costly to construct, it is inefficient to operate. An air-gas mixture firing system uses 30 to 75 cubic feet per hour of gas to heat a one-foot section of channel with an air-gas mixture. It requires about 10 cubic feet of air for combustion of 1 cubic foot of natural gas. The air must be heated from an ambient temperature to the same temperature as the exhaust gas stream. About 70 to 85 percent of the energy used heats the air to the exhaust gas temperature, leaving less than 15 to 30 percent of the energy to be transferred as available heat (i.e., heat available for the glass forming operation). Thus, an air-gas mixture firing system has minimum efficiency of combustion.
In addition to having a minimum efficiency of combustion, an air-gas mixture firing system is an inefficient means to heat the molten glass G. The flame temperature of an air-gas mixture burner in the air-gas mixture firing system reaches about 3500° F. However, the optical properties of the molten glass G and products of combustion limit the amount of radiant energy that penetrates the molten glass G. This causes the temperature gradient to be high vertically through the molten glass G. The only way to control the temperature distribution is to control the profile of the burners.
To overcome the deficiencies of an air-gas mixture firing system, the air-gas mixture burners have been replaced with concentric-type oxygen-gas mixture burners. A typical oxygen-gas firing system is supplied by BH-F)(ENGINEERING) LTD. of England. The system uses burners commonly referred to as oxygen-gas burners. Oxygen-gas burners use oxygen (e.g., typically 90 to 99 percent purity with an impurity being a combination of nitrogen and argon) in a high purity as an oxidant and fossil fuel for a combustible hydrocarbon supply. The oxygen-gas burner ignites the mixture of oxygen and gas at the point of ignition or combustion. The oxygen-gas burners are placed 4-5 inches apart, similar to the spacing to the air-gas mixture burners described above.
The oxygen-gas burners reduce CO2 and NOx emissions, making these burners more environmentally friendly and possibly reducing greenhouse gas taxes. Oxygen-gas burners fire more efficiently by reducing the waste gas stream and providing more available heat for use in the glass forming operation. This holds true because an oxygen-gas burner requires less volume (i.e., 2 cubic feet) for combustion of 1 cubic foot of natural gas. Consequently, exhaust gases (i.e., the stream of gas used to heat the oxygen-gas mixture) are reduced by about 73 percent. As a result, about 65 percent of the energy in an oxygen-gas mixture firing system is used to transfer available heat.
In addition to having a greater efficiency of combustion, an oxygen-gas mixture firing system is a more efficient means to heat the molten glass. The flame temperature of an oxygen-gas burner is about 4500 to 4800° F. At this temperature, the flame and products of combustion radiate energy at wavelengths that the molten glass can absorb. This provides uniform glass temperature horizontally on the surface of the molten glass and vertically through the molten glass.
Although an oxygen-gas mixture firing system provides uniform glass temperature, it requires an extensive number of complex and costly components. For example, the current cost of an oxygen-gas burner is about $1000. A conventional oxygen-gas mixture system uses about six oxygen-gas burners per foot, resulting in a cost of about $6,000 per foot.
What is needed is a front end that reduces fuel consumption by using a low-cost system for firing forehearths with a combination of gas and oxygen.